In the manufacture of paper and paperboard from cellulosic material, it is customary to use a sizing agent either at the wet end (known as internal sizing), or in the drying section of the papermaking machine, in order to increase the resistance of the paper or board to wetting and penetration by liquids, particularly aqueous liquids, and hence provide the cellulosic material with a degree of water repellency. Application of a sizing agent in the drying section of the machine is normally referred to as surface sizing (or external sizing) of the paper sheet or board. Various hydrophobic materials are used as external sizing agents, including rosin or rosin derivatives, paraffin waxes, synthetic resins and chemically reactive sizing agents, for instance alkyl ketene dimers. External sizing agents are usually applied by roll application at the size press in the drying section of the papermaking machine.
It has been proposed in British Patent Specification No. 1,039,540 to apply to a substantially dry paper sheet or board a liquid coating composition in the form of a foam in order to reduce the wetting effect thereon of the liquid medium present in the coating composition (and consequently the amount of subsequent drying necessary to return the paper sheet or board to its dry state) and thereafter mechanically to disintegrate the foam and form a continuous surface coating on the paper sheet or board. The coating composition may incorporate conventional coating materials, for example, china clay, starch, waxes, resins, rosin, titanium dioxide pigment or carboxymethylcellulose. In order to produce the foam, the coating composition must contain a foaming agent, for example, a surface active agent such as sodium lauryl sulphate. However, it is now known that even relatively low addition levels of surfactants that cause foaming, such as sodium lauryl sulphate, have a deleterious effect on the degree of sizing of paper sheet or board. It is probably principally for this reason that the coating method proposed in the aforementioned specification never achieved general acceptance in the paper industry, especially since in papermaking systems where recycling of paper and treatment materials takes place, any foamable surfactant present in the system would tend to build up in the wet end of the papermaking machine and consequently affect both internal and external sizing, and would have other deleterious effects, including a build-up of foam.
It is well known in the papermaking art that the presence of foam in the wet end of the system is generally to be avoided, particularly where recycling of paper or treatment materials occurs, because the foam affects the appearance and quality of the finished paper. Foam build-up can be reduced by addition of defoamer, (e.g. see Example 1 of British Patent No. 1,039,540), but this is only a partial solution and increases the manufacturing costs. The concentration of foamable surfactant would therefore have to be reduced by the release of some recirculating water to effluent and replacement with fresh water, which could lead to pollution problems.
Other patents which show the application of foam to a substrate are U.S. Pat. Nos. to Read, et al., 3,210,240 and Ashmus, et al, U.S. Pat. No. 4,023,526. Read, et al. indicate that paper can be "sized" using a composition that contains a conventional wetting agent or surfactant to aid the production of foam, the "sizing" material being starch. Ashmus, et al. mention treating compositions containing a conventional surface active agent as a foaming agent, functional or treating chemicals, wetting and foam-stabilizing agents, and water. The processes of these disclosures suffer from the disadvantages resulting from the aforementioned build-up of foam.